(1) Field of the Invention
The invention relates to the field of energy conversion and, more particularly, direct current-direct current voltage converters, well known to those skilled in the art and conventionally referred to just as DC-DC converters. The invention more specifically aims to limit the losses in a DC-DC converter comprising a plurality of interlaced cells, known by the name of interlaced multi-cell converter.
(2) Description of Related Art
A voltage converter conventionally comprises power cells each comprising switches which are controlled in such a manner as to chop an input voltage so as to form an output voltage of desired value.
In order to limit the switching losses, a known solution is to use a converter that comprises power cells comprising switches with controlled turn-off and soft start. With reference to FIG. 1, a power cell A comprises two switches K1, K2, connected in series, which are controlled in an alternating manner by a control device not shown. Each switch K1, K2 has a capacitor C1, C2 connected in parallel so as to delay the rise of the voltage and avoid the switching losses of the switches K1, K2. Each power cell A furthermore comprises an inductor L one end of which is connected between the two switches K1, K2 as shown in FIG. 1. The other end of the inductor, referenced S in FIG. 1, forms the output of the cell A and is connected to a load 1 which is supplied by the current Is output from the power cell A, whose value depends on the switching operation of the switches K1, K2.
In order to limit the switching losses, each cell switches at a voltage zero, known as ZVS for “Zero Voltage Switching”, with a Capacitor Minimum Discharge Current threshold, known as CMDC switching threshold, which must be exceeded in order to enable ZVS switching. Such a ZVS-controlled power cell with its CMDC switching threshold is known to those skilled in the art.
In order to obtain an output voltage of desired value for a ZVS voltage-zero switching mode, a known solution is to make the current in the coil L of the power cell A oscillate between an upper threshold M and a lower threshold N, the cell switching when the intensity of the current reaches one of the thresholds N, M at times Tn, Tm as shown in FIGS. 2 to 4.
As shown in FIG. 2, with a high load, the output current Is in the inductor L oscillates with high amplitude. Its average value Imf, of around 40 A-50 A, corresponds to the current which is consumed by the load 1, the slope of the oscillation depending on the value of the inductor L.
The electrical losses in the cell A are a function of the oscillation of the current Is. For high loads, the electrical losses are relatively high but are generally negligible compared to the output power supplied to the said load 1.
For low loads, as shown in FIG. 3, the values of the thresholds are modified in order to obtain a low-load average current Imb of around 1-10 A. The curve of output current Is in the inductor L is shifted downwards, the average value Imb corresponding to the current consumed by the load 1. For low loads, the output current consumed is not very high but the electrical losses associated with the oscillations of the output current Is remain constant. The energy efficiency is low.
In order to overcome this drawback, a known solution is to increase the switching frequency of the cell so as to limit the amplitude of the oscillations as shown in FIG. 4. In order to increase the switching frequency, a switching control device is known that is based on the hysteresis principle which allows the difference between the upper threshold M and lower threshold N to be varied as a function of the value of the load 1, in other words of the value of the output current Is consumed by the load 1. In other words, the higher the value of the load, the greater the difference between the upper threshold M and the lower threshold N.
For low loads, with reference to FIG. 4, the lower threshold value N and the upper threshold value M are close in accordance with the hysteresis principle. The output current Is oscillates with a lower amplitude but at a higher frequency, the value of the slope of the oscillation being constant owing to the fact that it depends directly on the value of the inductor L. Since the amplitude of the oscillations is lower, the electrical losses are less. Such an operation is satisfactory for a single-cell voltage converter.
In order to obtain a substantially continuous output current in the load 1, an interlaced multi-cell voltage converter is known that comprises a plurality of power cells each supplying a sinusoidal output current which is phase-shifted with respect to the other currents of the cells. Thus, the sum of the currents of the cells forms a “smoothed”, virtually continuous, overall current which improves the lifetime of the load receiving such an output current.
In order to limit the electrical losses with a low load for a multi-cell converter, an immediate solution would be to modify the switching frequency in the same manner as for a single-cell converter according to the hysteresis principle. However, this solution presents drawbacks associated with the dispersion of the values of the inductors L in the cells A. The problem is that, since the slope of the current intensity curve depends on the inductance L, the switching frequencies of the cells are different.
The result of this is that the output currents Is of the cells which are initially out of phase by the same phase difference period so as to form a smooth overall current, are phase-shifted with respect to one another over time. In the most critical case, the output currents of the cells can oscillate in phase. Thus, in contrast to the desired goal, a control device relying on the hysteresis principle results in large oscillations in the overall output current of an interlaced multi-cell converter.
Furthermore, for a low load, the switching frequency of the cells increases owing to the hysteresis, which increases the probability of in-phase oscillations of the output currents of the cells. The overall output current obtained then exhibits oscillations with high amplitudes which lead to significant electrical losses. An increase in frequency according to the hysteresis principle for an interlaced multi-cell converter does not allow the electrical losses to be limited, but quite the contrary.